Effect of pH on membrane morphology, fouling potential, and filtration performance of nanofiltration membrane for water softening

Negatively charged thin-film-composite polyamide nanofiltration (NF) membranes were used to filter aqueous calcium sulfate solution, examining potential for water-softening application. The effect of feed pH on deposit formation, fouling potential, and filtration performance of fouled membranes was systematically studied. The nature of the fouling layer formed on the membrane surface was found to be different at different feed pH values. A relatively sticky deposit that is difficult to discharge was obtained from an acidic feed solution, whereas a loose and easily removable deposit was formed from a feed solution of alkaline pH. Fouling potential was measured for solutions maintained at different pH values and was demonstrated to be dependent on the feed-solution pH. In the performance study of fouled membranes, a significant increase in the flux and unacceptable decrease in rejection were observed for membranes used in filtering acidic feed solutions. The fouled NF membranes were characterized using field emission scanning electronic microscopy (FESEM), atomic force microscopy (AFM), and attenuated total reflection Fourier transform infrared (ATR-FTIR) techniques to probe the possible reasons for the observed changes in the performance of the membranes. The FESEM study shows that the thickness of the selective top layer decreased with increasing acidity of the feed solution and, as evidenced from AFM study, the membrane surface became smoother compared to the clean membrane. The decrease in thickness of the top layer supports the finding of flux enhancement, and reduction in salt retention is believed to be due to the combined effect of reduced charge density on the membrane surface and decreased thickness of the selective layer, allowing enhanced ion transport through the membranes.